l. Field of the Invention
The present invention relates generally to digital bus structures and more particularly relates to a converter for converting between single-ended and differential bus signals.
2. Description of the Relevant Art
The Small Computer System Interface (SCSI) bus protocol is fully described in the American National Standard ANSI X3.131-1986 available from American National Standards Institute, Inc., 1430 Broadway, New York, NY 10018. Generally, up to eight devices, including small computers and peripherals, may be coupled to the bus. The bus includes data and control lines with the bus protocol facilitating information transfer between the devices on the bus. A particularly useful feature of this bus is that a separate bus controller is not utilized. The bus protocol is implemented by the devices coupled to the bus.
A short overview of some aspects of the protocol follows. A complete description is provided in the above-referenced standard, while more detailed examples of particular aspects of the protocol are provided below.
Information transfers are initiated by a computer, with the actual computer initiating a given transfer denoted the initiator. The initiator selects a target which controls the actual information transfer. In certain situations an information transfer may be interrupted and at a later time the target will reselect the initiator to continue the transfer.
The physical bus includes a set of conductive lines for carrying electrical bus signals and terminators at both ends of the bus. Devices coupled to the bus cannot be remote from the bus unless a special converter is coupled to the bus at one side and a cable to the remote device at the other side. The converter functions to satisfy the electrical requirements of the bus standard and includes receiver/driver pairs to interface the required signals to the remote device.
A particularly useful type of converter converts between single ended signals transmitted on an internal bus and differential signals transmitted on an external bus. These differential signals are less sensitive to noise and are required by some data storage devices. Ideally, the converter would be transparent to devices coupled to either bus and the combination of both buses would function as a single bus.
However, the SCSI protocol creates situations where the receiver/driver pairs converting between single-ended and differential signals become latched and cannot be released by the device asserting the signal. Accordingly, converters may be utilized only for configurations that do not utilize the full capability of the SCSI protocol. Thus, a converter providing for a more complete use of this capability is greatly needed because of the wide acceptance of the standard and the great number of devices being manufactured to interface with the SCSI bus.